Hi kkcci88888,
Per my understanding that you have configured that when the user access report manager will pop up windows asked for credetial like below, but now the credential pop up window didn't pop up, right?
By default, we will configure the authentication type as "<RSWindowsNTLM/>" which will not pop up any login windows when access report server and report manager.
If you have develop any custom authentication or use IP instead of ServerName in the Report Manager URL, it will pop up login window ask for username and password.
I would like to know what settings you have done to make the login window pop up? Please try to provide more details information.
Regards,
Vicky Liu